ASTR 121 (O'Connell) Study Guide 20
Outer Plan Title

Jupiter from Voyager with Io and Europa in foreground


"And now for something completely different," as they used to say on Monty Python.

The Jovian planets (Jupiter, Saturn, Uranus, and Neptune) are entirely unlike the terrestrial planets. They may have rocky cores, like larger versions of the Earth, at their centers, but these are enveloped in giant gaseous atmospheres. Only the outermost skins of these atmospheres can be studied directly. This is meteorology, instead of the geology/topography we discussed for the terrestrials. However, it can be just as extreme with respect to Earth-bound meteorology as are the canyons and mountains of Mars compared to those of Earth.

Another major distinction of the Jovians is the large number of satellites they possess. The satellites, observed at close range by spacecraft, exhibit an astonishing diversity of surface types and features. Unlike the three terrestrial planet satellites, the larger Jovian satellites are rich in water ice and exhibit many different phenomena as a consequence. In many ways, they are more interesting than their parent planets. They may even harbor biospheres. The ring systems of the Jovians are probably the remnants of distintegrated satellites.

Many examples of a third kind of planet have recently been discovered outside the orbit of Neptune. These are perhaps most aptly called the "ice dwarfs," of which Pluto is the archetype.


A. HISTORY



B. JOVIAN PLANETS (J,S,U,N): PROPERTIES

These four share gross properties. Pluto is entirely different (see below).

Distant from Sun: 5-30 AU. (Pluto is at 39 AU.) The outer solar system is vast (over 10,000 times the volume of the inner solar system out to Mars) and sparsely populated.

Large: 4-11 x Earth radius. Masses 15(U)-318(J) x Earth. J. contains twice as much mass as all other planets combined. An animated image of Jupiter's rotation is shown at the right.

Structures

Visible Surfaces

Special Probes of Jupiter

Magnetic Fields: strong; generated by motions in liquid metallic hydrogen interior in J,S; produce strong radiation belts, up to 100x Earth's



Pseudo-color infrared image of Saturn

C. RING SYSTEMS

Saturn system is brightest, but rings are present around all 4 Jovians

Saturn's Rings


D. JOVIAN PLANET SATELLITES

Numerous: 8-63

Diverse(!) characteristics; often violent histories

Larger moons are mixtures of rocky/icy materials

Smaller sats irregular in shape

Interesting examples:

Artist's Concept of Huygens Probe at Titan


E. PLUTO AND THE KUIPER BELT

Pluto is entirely unlike the other four large outer planets. It is smaller by a factor of 2 than any of the other 8 planets. It is a rocky/icy object rather than a gas giant.

When first discovered, Pluto was thought to be isolated at the edge of the Solar System. However, in the last 15 years, astronomers have discovered many more such bodies, some with sizes comparable to Pluto. These are all members of the "Kuiper Belt".

These discoveries precipitated the messy discussion at the International Astronomical Union in the summer of 2006. Astronomers held a debate over the meaning of the term "planet"---specifically whether or not Pluto and the other large KBO's should be placed in a separate category. In the end, the IAU voted to create a new category of "dwarf planet" for these latter objects but was then forced to add the asteroid Ceres for consistency. All this was handled very clumsily, and it generated needless controversy. It turns out many non-astronomers were fond of Planet Pluto.

Even before the discovery of Pluto, we had already known of many small, rocky objects in separate orbits around the Sun---the "asteroids." Now, we know about many similar, but icy, objects. Sensible designations for these types, above some threshold in size, are as "rock dwarf planets" and "ice dwarf planets."


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Last modified April 2008 by rwo

Text copyright © 1998-2008 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.